In exhaust gas cleaning systems of a vehicle, exhaust gas sensors (air/fuel ratio sensors or oxygen sensors) for detecting an air/fuel ratio or rich/lean of exhaust gas are installed on an upstream side and a downstream side of a catalyst for cleaning the exhaust gas. The air/fuel ratio is feedback-controlled based on outputs of the exhaust gas sensors to thereby promote an exhaust gas cleaning efficiency of the catalyst.
In the exhaust gas cleaning systems, a deterioration in the exhaust gas sensors is diagnosed in order to prevent operation from being continued in a state in which accuracy of the air/fuel ratio control is lowered (state in which an exhaust gas cleaning rate is low) by deteriorating the exhaust gas sensors. According to the method of diagnosing the deterioration of the exhaust gas sensors, generally, presence or absence of the deterioration of the exhaust gas sensors is determined by whether a change of the outputs of the exhaust gas sensors follow a change in the air/fuel ratio with excellent response when the air/fuel ratio is changed.
However, the change of the outputs of the exhaust gas sensor installed on the downstream side of the catalyst is influenced by a cleaning function of the catalyst (storage effect) Therefore, a delay occurs until the change in the air/fuel ratio (target air/fuel ratio λTG) on the upstream side of the catalyst emerges as a change in the air/fuel ratio on the downstream side of the catalyst (output of exhaust gas sensor). The delay time period is changed by the cleaning function and a degree of deterioration of the catalyst at the time point. Therefore, when the deterioration of the exhaust gas sensor is diagnosed based on the change of the output of the exhaust gas sensor on the downstream side of the catalyst, the change of the output of the exhaust gas sensor on the downstream side of the catalyst is influenced to change by the cleaning function of the catalyst (storage effect) at the time point and presence or absence of the deterioration of the exhaust gas sensor on the downstream side of the catalyst cannot accurately be determined.
Hence, as shown by JP-A-9-170966, a time period during which an output of an oxygen sensor on a downstream side of a catalyst is changed from a rich side set value to a lean side set value at each cutting of fuel is measured as a response time period. Presence or absence of deterioration in the oxygen sensor on the downstream side of the catalyst is determined by whether the response time period is equal to or longer than a deterioration determinant (primary diagnosis). As a result, in the case in which the determination is determined to be present, when cutting off fuel is carried out continuously for a predetermined time period, at a time point at which an elapse time period after recovery from cutting off fuel reaches a set time period, a minimum response time period which has been measured until the time point is read from a memory to compare with the deterioration determinant and when the response time period is determined to be equal to or longer than the deterioration determinant again, the oxygen sensor on the downstream side of the catalyst is firmly diagnosed to be deteriorated.
According to the apparatus, when diagnosing the deterioration of the oxygen sensor on the downstream side of the catalyst, an influence of the storage effect of the catalyst is disregarded by cutting off fuel. That is, by utilizing a property that the response time period from starting to cut off fuel until the air/fuel ratio on the down stream side of the catalyst is changed to be lean since in cutting off fuel, a large amount of lean component (O2or the like) flows into the catalyst and an amount of adsorbing the lean component in the catalyst is rapidly brought into a saturated state, the deterioration of the oxygen sensor is diagnosed by measuring the response time period of the oxygen sensor on the downstream side of the catalyst in cutting off fuel.
However, actually, the response time period of the oxygen sensor on the downstream side of the catalyst is changed by the storage effect of the catalyst. That is, as shown in FIG. 5, when the air/fuel ratio on the upstream side of the catalyst is switched from rich to lean by cutting off fuel, in the midst of changing the air/fuel ratio on the downstream side of the catalyst (output of oxygen sensor) is being changed from rich to lean, the air/fuel ratio on the downstream side of the catalyst hardly changes temporarily by the storage effect of the catalyst. The more progressed is the degree of deterioration of the catalyst, the shorter a duration time period of the storage effect and the response time period of the oxygen sensor on the downstream side of the catalyst is shortened. Therefore, in diagnosing the deterioration of the oxygen sensor on the downstream side of the catalyst, the influence of the storage effect of the catalyst cannot be disregarded and presence or absence of the deterioration of the oxygen sensor on the downstream side of the catalyst cannot accurately be determined.
Further, when the air/fuel ratio is changed in operating an engine for diagnosing the abnormality of the exhaust gas sensor, adverse influence is effected on exhaust emission or drivability.
Further, when the abnormality diagnosis is carried out after waiting for a change in the air/fuel ratio under a predetermined condition by cutting off fuel, depending on a method of operating a vehicle or a road situation, chances of changing the air/fuel ratio under the predetermined condition are reduced and the frequency of carrying out the abnormality diagnosis is reduced and the abnormality of the exhaust gas sensor may not be detected at early time. Further, when the abnormality diagnosis is carried out by forcibly changing the air/fuel ratio on the upstream side of the catalyst (target air/fuel ratio) in operating the engine in order to ensure the frequency of carrying out the abnormality diagnosis, a running function or an exhaust cleaning function of the catalyst may be deteriorated.